.main{
    text-align: center;
}
.side{
    background-color: red;
    width: 300px;
    height: 100px;
    transition: all 1s;
}
.window{
    width: 100px;
    height: 100px;
    border: 3px solid black;
    z-index: 1;
    margin: 30px auto;
}